Blizzard's Bashiok fills us in on how D3 Will allow players to play Single Player online and cooperative at the same time.

No. Not that classes won't benefit from each other in various ways, Barbarian shouts being good party buffs, etc. but every class is essentially a demon-killing DPS class. We have no intentions to create a tank, or healer, etc. Every class will play their own way, and while some may take direct hits easier than others, it certainly wouldn't define their role within a party. Everyone should be kicking in heads at the same time.

Diablo III having a strong emphasis on cooperative play is really building upon one of the best parts of Diablo II, and that's charging through dungeons with other people. Our focus is on removing the frustrations that existed with grouping, and adding new features that encourage everyone to stick together and fight as a party.